RFC 4122 is a document that specifies a standard for generating Universally Unique Identifiers (UUIDs). UUIDs are 128-bit values used to uniquely identify information in computer systems, ensuring that identifiers are unique across different contexts and systems. This standard is widely used in software development, databases, and network protocols. The document outlines several versions of UUIDs, each with different methods for generation. For example, Version 1 UUIDs are based on the current time and the machine's MAC address, while Version 4 UUIDs are generated randomly. This flexibility allows developers to choose the most suitable method for their applications.